1. Field of the Invention
The present invention relates to an optical pickup apparatus using a laser beam to perform an action of reading out signals recorded on an optical disc and an action of recording signals onto the optical disc.
2. Description of the Related Art
An optical disc apparatus has become widespread that is capable of performing actions of reading out signals and recording signals by irradiating a laser beam emitted from an optical pickup apparatus onto a signal recording layer of an optical disc.
Although a generally widespread optical disc apparatus is of a type using an optical disc called a CD or a DVD, there has been recently developed a type using an optical disc with an improved recording density, i.e., a Blu-ray standard optical disc.
An infrared beam with a wavelength of 785 nm is used as a laser beam for performing an action of reading out signals recorded on a CD standard optical disc, while a red beam with a wavelength of 655 nm is used as a laser beam for performing an action of reading out signals recorded on a DVD standard optical disc.
The thickness of a transparent protective layer provided between a signal recording layer included in the CD standard optical disc and a surface of the optical disc is 1.2 mm, and the numerical aperture of an objective lens used to perform the action of reading out signals from the signal recording layer is set to 0.47. The thickness of a transparent protective layer provided between a signal recording layer included in the DVD standard optical disc and a surface of the optical disc is 0.6 mm, and the numerical aperture of an objective lens used to perform the action of reading out signals from the signal recording layer is set to 0.6.
A laser beam with a short wavelength, e.g. a blue-violet beam with a wavelength of 405 nm is used as a laser beam for performing an action of reading out signals recorded on the Blu-ray standard optical disc, in respect to the CD standard and DVD standard optical discs.
The thickness of a protective layer provided on an upper surface of a signal recording layer included in the Blu-ray standard optical disc is 0.1 mm, and the numerical aperture of an objective lens used to perform the action of reading out signals from this signal recording layer is set to 0.85.
It is required to reduce the diameter of a laser spot generated by condensing a laser beam in order to perform the action of reading out signals recorded on the signal recording layer included in the Blu-ray standard optical disc and the action of recording signals onto the signal recording layer. The objective lens used to obtain a desired laser spot shape is characterized in that its radius of curvature becomes small because not only does its numerical aperture increase but also its focal length decreases.
Although there is commercially provided an optical disc apparatus capable of performing actions of reading out signals recorded on all optical discs of the above-described CD standard, DVD standard, and the Blu-ray standard and actions of recording signals thereonto, an optical pickup apparatus assembled in such an optical disc apparatus incorporates a laser diode that emits laser beams having wavelengths corresponding to each of the above standards and an objective lens configured to condense laser beams emitted from the laser diode onto a signal recording layer included in each optical disc.
The optical pickup apparatus capable of performing the actions of reading out signals recorded on the optical discs of all the different standards described above incorporates two objective lenses, one for performing an action of condensing a laser beam applied to the optical discs of the CD standard and the DVD standard, and the other for performing an action of condensing a laser beam applied to the optical disc of the Blu-ray standard.
The optical pickup apparatus incorporating the two objective lenses poses not only a problem of an optical system configuration becoming complicated but also a problem of the shape of the optical pickup apparatus becoming large. As a method for solving the problems, a technique has been developed that enables a single objective lens to condense laser beams onto the optical discs of all standards.
The optical pickup apparatus involves a problem that it cannot perform normal actions of reading out and recording signals as a result of occurrence of a spherical aberration attributable to the thickness of the protective layer interposed between a disc surface that is a laser beam incident surface of the optical disc and the signal recording layer thereof. As a method for solving such a problem, a technique has been developed to correct the spherical aberration by displacing a collimating lens provided between the laser diode and the objective lens along its optical axis direction (refer to Japanese Patent Application Laid-Open Publication Nos. 2006-236414 and 2004-14042).
Although the optical pickup apparatus described in Japanese Patent Application Laid-Open Publication No. 2006-236414 is configured to use a single objective lens to perform the actions of reading out signals recorded on the three types of optical discs of different standards, the actions of selecting the laser beams and setting the numerical aperture of the objective lens are carried out by a diffraction optical element disposed at the incident surface side of the objective lens, so that the number of optical system components increases. As a result, there arises a problem that the assembling and adjusting actions cannot easily be performed.
Another problem also arises of a shortage in the quantity of light of the laser beam used for the action of reading out signals recorded on a first optical disc, due to such a configuration that laser beams incident on the outer circumference side of the objective lens are used to read out signals recorded on the first optical disc and that laser beams incident on the inner circumference side thereof are used to read out signals recorded on a second optical disc and a third optical disc.